1. Field of the Invention
The present invention relates, in general, to compositions for extinguishing fires. More particularly, the present invention relates to biodegradable foam compositions that are capable of extinguishing both hydrocarbon-based fires and water-soluble fuel-based fires.
2. Statement of the Problem
Hydrocarbon-based products, for example, crude oils and products derived from crude oils such as gasoline, jet fuels, etc., are extremely flammable. Fires involving such hydrocarbons sometimes occur, and when large amounts of such hydrocarbons are stored in one place (fuel bunkers or oil tanks), these fires can be extremely large and difficult to extinguish.
The use of foams to extinguish hydrocarbon-based fires has long been known. Foams generally extinguish such fires by smothering them, that is, preventing oxygen from reaching the combustible materials. Several types of foams have been used to extinguish hydrocarbon-based fires. For example, prior to about the mid-1960s, protein foams were used for this purpose. These foams are formulated with hydrolyzed protein, for example, hydrolyzed keratin, albumins and globulins. Typically such foams are also provided with ferrous sulfate to help provide a foaming action that is particularly useful for extinguishing hydrocarbon-based fires. However, these protein-based foams are not always effective--often because their use requires that a uniformly applied, heavy blanket of foam be applied over the entire fire. Any disruption in the integrity of these foams often results in a flare-up of the burning fuel. These hydrolyzed protein foaming agents also suffer from the disadvantage of having relatively short shelf lives.
In the mid-1960s, aqueous film-forming foams (AFFF) were developed. AFFF are less dense than protein foams and operate by spreading an aqueous film on the surface of hydrocarbon liquids, thus enhancing the speed at which fires involving such liquids can be extinguished. The aqueous film produced by AFFF results from the use of fluorochemical surfactants as ingredients. These fluorosurfactants produce very low surface tension values (15-20 dynes per cm) that permit AFFF using them to quickly spread as an aqueous film on the surface of hydrocarbon liquids. Unfortunately, fluorocarbons such as these are known toxicants and are extremely difficult to remove by biodegradation, either natural or accelerated. Thus, extinguishing a fire with compositions containing fluorocarbons leaves a toxic residue.
AFFF also may require frequent reapplication since any breaks in the fragile surfactant film over the combustible material sometimes allows the combustible material to reignite. This drawback led to development of various AFFF having improved barrier properties in the aqueous film. For example, U.S. Pat. No. 5,085,786 discloses an improved AFFF containing fluoroaliphatic amphoteric surfactants, fluoroaliphatic anionic surfactants, and short-chain alkyl ether sulfate hydrocarbon surfactants. These improvements notwithstanding, the use of AFFF still presents problems, and there remains a need for AFFF whose aqueous film barriers are less likely to break down, especially in fighting fires involving three-dimensional structures. Moreover, AFFF are not very effective in fighting fires involving water-soluble fuels such as alcohols. If AFFF are used on fires involving such fuels, they tend to be quickly dissolved and destroyed by the fuel itself.
Consequently, other types of foam have been developed to fight fires involving water-soluble fuels. They are called alcohol-resistant AFFF, or ARAFFF. In addition to the ingredients employed in AFFF, ARAFFF contain a water-soluble polymer that precipitates on contact with a water-soluble fuel and thereby provides a protective layer between the water-soluble fuel and the foam. Many ARAFFF have also proven effective in extinguishing fires of both hydrocarbons and water-soluble fuels. Again, ARAFFF are similar to AFFF in that they contain, in addition to a water-soluble polymer, one or more perfluoroalkyl surfactants that may be anionic, cationic, or nonionic, solvents such as glycols and/or glycol ethers. They also usually contain minor amounts of additive-type ingredients such as chelating agents, pH buffers, corrosion inhibitors, and the like.
ARAFFF were first disclosed in U.S. Pat. No. 4,060,489. This patent describes a foam containing a fluorocarbon surfactant and a silicone-containing sulfated surfactant, an imidazoline surfactant, a thixotropic polysaccharide such as scleroglucan (a polymeric form of glucose) or xanthan gum, N-methyl pyrrolidone-2 (a viscosity enhancer), ethylene glycol, and a foam-stabilizing hydrophilic resin. Other useful ARAFFF are disclosed in U.S. Pat. No. 4,306,979 to Tsuji et al., U.S. Pat. Nos. 4,999,119 and 5,207,932 to Norman et al., U.S. Pat. No. 5,391,721 to Hanen et al., and U.S. Pat. No. 5,496,475 to Jho et al.
The most common ingredient(s) in all AFFF and ARAFFF that have been developed to date are perfluoroalkyl surfactants. Unfortunately, these surfactants are known toxicants that are extremely difficult to remove from the environment once they have entered it. Thus, extinguishing a fire with currently known AFFF or ARAFFF leaves a nearly permanent, highly toxic perfluoroalkyl residue. Such perfluoroalkyl surfactants also usually represent up to 80% of the cost of an AFFF or ARAFFF concentrate. It therefore would be very desirable to reduce or eliminate perfluoroalkyl surfactant ingredients from fire-fighting foams for ecological as well as economic reasons if equally effective, and less costly, fire-fighting agents were available.
To this end, U.S. Pat. No. 5,207,932 (the '932 patent) discloses certain AFFF and ARAFFF in which perfluoroalkyl surfactants have been reduced in concentration by more than 40% without loss of fire-fighting performance. This is achieved by using alkyl polyglycoside surfactants in such compositions. For example, the '932 patent discloses an AFFF concentrate comprising a perfluoroalkyl surfactant, a solvent, and an alkyl polyglycoside. Its ARAFFF concentrates are comprised of a perfluoroalkyl surfactant, a solvent, an alkyl polyglycoside, and a water-soluble polymer. All embodiments of the '932 patent, however, still call for a perfluoroalkyl surfactant. Thus, although this invention helps to reduce the cost of AFFF and ARAFFF because of its call for reduced amounts of relatively expensive perfluoroalkyl surfactant ingredients, such surfactants are still contained in these compositions; hence, their use still implies environmental pollution.
It also should be noted that ARAFFF concentrates are typically diluted to different concentrations for use on different types of fires. For fires involving hydrocarbon liquids, ARAFFF concentrates are diluted at the time of application to a 3% concentration (that is, 3 parts concentrate to 97 parts water). Fires involving water-soluble fuels, however, require an ARAFFF concentration of 6% (6 parts of concentrate to 94 parts water.). This implies extra expense because of the larger amounts of concentrates needed for fires of this type. Some ARAFFF concentrates can be dilutely formulated for application to water-soluble fuel fires, e.g., those 3% solutions taught in U.S. Pat. No. 5,496,475; however, such ARAFFF are impractical to use because of their extremely high viscosities. For example, the prior art has found that in order to use a 3% dilution of ARAFFF, the amount of polysaccharide gums they contain must be reduced to lower the viscosity of the concentrated solution. However, this decrease in the amount of gums results in a composition that is decidedly less effective at extinguishing fires.
Several attempts have been made to lower the viscosity of ARAFFF without reducing the amount of polysaccharide gums in order to provide compositions that are effective at 3% strength. For example, U.S. Pat. Nos. 4,999,119 and 5,207,932 disclose the use of alkyl polyglycosides to help reduce the amounts of viscosity-enhancing polysaccharides. In the same vein, U.S. Pat. No. 5,496,475 teaches use of anionic copolymers such as methacrylic acid-acrylamide-methacrylate or methacrylic acid-N,N-dimethylacrylamide to reduce the viscosity of ARAFFF. Such compositions still however contain fluorosurfactants, hence their use still implies all the negative environmental and economic consequences previously noted.
It would be advantageous, therefore, to provide fire-fighting foam concentrate compositions that do not contain any fluorocarbon surfactants whatsoever and that can be applied at less than a 3% dilution and yet contain high concentrations of polysaccharide gums in order to provide more effective fire-fighting capabilities against both hydrocarbon and water-soluble fuel fires.